A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
In a lithographic process (i.e., a process of developing a device or other structure involving lithographic exposure, which may typically include one or more associated processing steps such as development of resist, etching, etc.), it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes (SEM), which are often used to measure critical dimension (CD), and specialized tools to measure overlay (OV) (i.e., the accuracy of alignment of two layers in a device). Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle or a range of angles of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
Fabrication tolerances continue to tighten as semiconductor devices become ever smaller and more elaborate. Hence, there is a need to continue to improve metrology measurements. One exemplary use of scatterometers is for critical dimension (CD) metrology, which is particularly useful for measuring in patterned structures, such as semiconductor wafers. Optical CD metrology techniques include on dome scatterometry, spectral reflectometry, and spectral ellipsometry. All these techniques are based on measuring the reflected intensity of differently polarized light for different incident directions. Such techniques require a high extinction ratio, or purity of polarization. A polarizing beamsplitter (PBS) divides light by polarization state to transmit p-polarized light while reflecting s-polarized light. Though a perfect PBS transmits 100% of the p-polarization and reflects 100% s-polarization, a real PBS transmits and reflects mixtures of s-polarized light and p-polarized light. The ratio between the p-polarized light and s-polarized light is called the extinction ratio. Optical CD requires a high extinction ratio.
Another exemplary use of scatterometers is for overlay (OV) metrology, which is useful for measuring alignment of a stack of layers on a wafer. In order to control the lithographic process to place device features accurately on the substrate, alignment marks, or targets, are generally provided on the substrate, and the lithographic apparatus includes one or more alignment systems by which positions of marks on a substrate must be measured accurately. In one known technique, the scatterometer measures diffracted light from targets on the wafer. Diffraction-based overlay using “dark field” scatterometry blocks the zeroth order of diffraction (corresponding to a specular reflection), and processes only one or more higher orders of diffraction to create a gray scale image of the target. Diffraction-based overlay using this dark field technique enables overlay measurements on smaller targets, and is known as micro-diffraction-based overlay (μDBO). μDBO, however, requires a very high contrast ratio.
Each product and process requires care in the design of metrology targets and the selection of an appropriate metrology ‘recipe’ by which overlay measurements will be performed. In a known metrology technique, diffraction patterns and/or dark field images of a metrology target are captured while the target is illuminated under desired illumination conditions. These illumination conditions are defined in the metrology recipe by various illumination parameters such as the wavelength of the radiation, its angular intensity distribution (illumination profile) and its polarization. The inspection apparatus includes an illumination system comprising one or more radiation sources and an illumination system for the delivery of the illumination with the desired illumination parameters. In practice, it will be desired that the illumination system can switch between different modes of illumination by changing these parameters between measurements.
Illumination profiles can be varied greatly and the use of custom illumination is becoming more and more important in optical metrology. The customization of the illumination enables improvement of the measurement quality. To customize the intensity across a pupil plane, aperture plates provide a certain measure of control in matching the mode of illumination to the metrology techniques. A filter wheel including a plurality of different apertures may be used to select a particular illumination mode. The filter wheel, however, is finite in size and, thus, can only accommodate a limited number of different apertures. In addition, the apertures on a filter wheel are static and, thus, do not allow adjustment of the individual apertures. Alternative approaches, such as using spatial light modulators (SLM) can increase flexibility, but have their own limitations. For example, transmissive liquid crystal (LC) SLMs or micro mirror arrays cannot achieve the extreme contrast required for μDBO. Also, known LC SLM arrangements may not provide the high extinction ratio needed for CD metrology. Further, use of multiple LC SLMs increases cost and complexity and create additional synchronization and calibration issues.